The present invention relates to a method for regenerating a point-to-multipoint interface on a point-to-point interface, especially in an ATM (Asyncronous Transfer Mode) design.
More specifically the present invention gives instructions for how to emulate a point-to-multipoint interface on a point-to-point interface and to regenerate the physical layer information that was lost when information was transferred from a point-to-multipoint interface to a point-to-point interface.
Today there exist several ATM-components on the market. Some have only point-to-point interfaces, while others have point-to-multipoint interfaces. Typically point-to-point interface are UTOPIA level 1 or SCI-PHY+ for single-PHY interfaces. The point-to-multipoint interfaces are typically UTOPIA level 2 or SCI-PHY+ for multiple-PHY (Ref. [1]) and Ref. ([2]).
This invention can typically be used when one have several physical interfaces (PhyI(1) to PhyI(n)) connected to ATM layer circuit, for example one policing-circuit, which has only one point-to-point interface towards a switch-port. If the switching-circuit has a point-to-multipoint interface capability, this solution can be inserted between the policing circuit and the switch port, so as to emulate a multi-point interface towards the switch port.
The benefit of this is to increase the number of connections per interface and to allow free use of VPIs per physical interface.
This invention covers the shaded part of FIG. 1.
Two cells with identical VPI and VCI can come into this system from different physical interfaces. When these two interfaces are merged into a point-to-point interface, these two cells must be separated in some way, and the difference must be visible to the switch-port. Some circuits have the possibility to insert a tag-field which may contain information about which physical interface the ATM-cell comes from. Circuits which are compliant to standarized point-to-multipoint interfaces (e.g. UTOPIA level 2) are, however, not able to use this information.
To distinguish between two cells with identical VPI and VCI on different physical interfaces, an address-translation in the ATM-cell header can be done in the circuit ahead of the switch-port. The switch-port will see two different ATM-cells on one physical address. The maximum number of VPCs in the switch-port will be limited to 4k (NNI), and the maximum number of VCCs will be 64k. If there had been a point-to-multipoint interface all the way into the switch-port, this limitation would have been per physical interface, but when using an address-translation this limitation is on all the physical interfaces together. The maximum number of VPCs on each physical interface will typically be 4096 (VPI is 12 bits wide) divided by the-number of physical interfaces.
This approach requires that the switch-port can be set in a point-to-point interface mode. Often, that can result in that the point-to-point interface on the transit-side also is disabled, which introduces limitations out from the switch-port as well.
From U.S. Pat. No. 5,541,915 (Storm) there is known a method for point-to-multipoint connections in self-routing ATM-switching networks, without limiting the dynamics thereof. This specification suggests further marking in the header data of the ATM-cells beyond VCI/VPI for multi-point transmission of ATM-cells without substantially increasing the volume of data.
A similar system is disclosed in U.S. Pat. No. 5,287,530 (Davis et al.) which discloses a multicast server operators operative to effect the onward transmission of data cells to a plurality of different addresses. When a data cell is received at one of a plurality of input ports, comprising an address which is indicated by the VCI and VPI as being a cell which should be routed to a plurality of different addresses, routing tags are set within a switch function unit such that the cell is routed to a transfer port. From U.S. Pat. No. 5,459,724 (Jeffrey et al.) there is known an ATM-switching arrangement comprising an input adaption port X with intermediate storage of cells and separated central control.
From U.S. Pat. No. 5,202,885 (Schrodi et al.) there is known an ATM system comprising switches for copying ATM data cells, so as to enable handling of various types of point-to-multipoint traffic, for example broadcast or multicast. This prior art ATM system comprises point-to-point connections having either high or low fanout. The copying of cells makes it possible for the point-to-point connections and for the system thereby to handle all types of data traffic.
However, U.S. Pat. No. 5,202,885 is silent about the problems which arise when ATM traffic on a card is transferred from a point-to-point interface, and is also silent about regenerating a point-to-multipoint interface on a point-to-point interface.
In other words, U.S. Pat. No. 5,202,885 relates to ATM connections referring to higher protocol layers and the connectivity between different types of end points, whereas the present invention relates to interfacing, i.e. the physical connection between circuits. Consequently, U.S. Pat. No. 5,202,885 is also silent about the generation of information which is lost at tahe transition between point-to-multipoint and point-to-point interfaces.
U.S. Pat. No. 5,418,786 (Loyer et al) and U.S. Pat. No. 5,485,456 (Shtayer et al) relate to additions and variations of interfaces of UTOPIA standards applicable between the physical layer and the ATM layer.
CA 2,181,293 (PCM-Sierra, INC.) relates to a policing circuit including interface towards physical layer, with the possibility of adding tag fields towards the switch gate.
The present invention relates to a method as stated in the preamble, which in a more efficient manner is able to regenerate a point-to-multipoint interface on a point-to-point interface, as compared with prior art.
Consequently, a method as stated in the preamble is according to the present invention characterized in that the method comprises the following steps:
a) using a circuit means with a point-to-point interface for the connection thereof to a plurality of interfaces,
b) using said circuit means to identify from which physical interface any cell originated, namely
c) by using pre-pending byte(s) or the user defined byte(s) in the respective ATM-cell, for thereby regenerating the physical layer information which was lost at transition between said interfaces.
Appropriately, the present method suggest that said pre-pending byte(s) or user defined byte(s) in said ATM-cells are assigned individual tags, for example a PID tag (Physical Interface Identification tag).
A further advantage of this method is that the use of said circuit means to identify from which physical interface any cell originated, is effected without introducing limitations on the number of VPCs (Virtual Path Connections) and/or VCCs (Virtual Channel Connections).